Warming climate will bring more extreme 'El Nino' events

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Rising global temperatures are likely to double the frequency of the most severe El NiÃ±os - the periodic atmospheric disruptions which affect weather across the globe. Tim Radford reports
An El NiÃ±o is part of a natural cycle: a huge blister of heat in the equatorial Pacific, usually around Christmas-time.

Rising global temperatures are likely to double the frequency of the most severe El NiÃ±os - the periodic atmospheric disruptions which affect weather across the globe. Tim Radford reports.

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An El NiÃ±o is part of a natural cycle: a huge blister of heat in the equatorial Pacific, usually around Christmas-time.

The phenomenon periodically triggers unseasonal floods in the western US, and extreme heat and forest fires in the Indonesian rainforest and the Australian bush.It happens and seems to have happened through human history. It has nothing to do with global warming or climate change.

Except this: according to the latest study by climate scientists in Australia, the US, China and Britain, global warming is likely to make the most extreme El NiÃ±o events happen twice as frequently.

Right now, and for the past year, conditions in the equatorial Pacific have been neither unusually warm nor unusually cool. There is no El NiÃ±o right now.But for two summers running, even without help from unusual Pacific conditions, Australia has been hit by record temperatures and appalling forest fires, so the news is ominous.

Wenju Cai of Australia's CSIRO marine and atmosphere research and colleagues report in the journal that extreme El NiÃ±o events tend to happen when sea surface temperatures higher than 28Â°C develop in the normally cool and dry eastern Pacific.

This can trigger big shifts in the atmospheric convection zones - areas of instability caused by temperature differences. These episodes have normally occurred every 20 years or so.

Profound impact

Now, as carbon dioxide levels rise and the global average temperatures creep up, these extreme events are likely to be twice as frequent: every decade or so.The last extreme event, in 1997-98, caused an estimated $35 billion in damage and claimed an estimated 23,000 lives worldwide. It also made 1998 the hottest year ever in average global temperatures, a record that lasted for more than a decade.